Fluid dynamic bearing unit and disk drive device including the same

ABSTRACT

A first sleeve rotatably extends around a shaft. First and second flanges are fixed to the shaft. A second sleeve extending around the first sleeve is fixed thereto. A first annular member fixed to the second sleeve surrounds the first flange. A second annular member fixed to the second flange surrounds a portion of the second sleeve. A first capillary seal includes a clearance between the first flange and the first annular member. A second capillary seal includes a clearance between the second annular member and the second sleeve. Lubricant is provided in the clearances in the first and second capillary seals. The second annular member and the second sleeve are designed so that the lubricant in the clearance in the second capillary seal can be viewed from a point in a radial position which is outward of the second sleeve as seen in an axial direction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 14/029,889,filed Sep. 18, 2013, which in turn is a divisional of application Ser.No. 13/891,243, filed May 10, 2013, now U.S. Pat. No. 8,564,908, whichin turn is a divisional of application Ser. No. 13/629,711, filed Sep.28, 2012, now U.S. Pat. No. 8,482,881, which in turn is a divisional ofapplication Ser. No. 13/332,851, filed Dec. 21, 2011, now U.S. Pat. No.8,441,759, which in turn is a divisional of application Ser. No.12/453,208, filed May 1, 2009, now U.S. Pat. No. 8,107,195.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a fluid dynamic bearing unit and a drivedevice for a recording disk. In addition, this invention relates to amethod of manufacturing a fluid dynamic bearing unit.

2. Description of the Related Art

In recent years, disk drive devices such as HDDs have included fluiddynamic bearing units (abbreviated to FDBs) to enhance disk driveperformances. Generally, a disk drive device capable of rotating arecording disk at a higher speed is desired. In a conventional diskdrive device for magnetically recording data on a recording disk, thespeed of rotation of the disk is 3600 min⁻¹. Recent disk drive devicesrotate recording disks at a speed of 5400 min⁻¹ or 7200 min⁻¹.

In a disk drive device, higher-speed rotation of a recording disk causesgreater unwanted vibration of a magnetic head which disturbs the tracingof a recording track by the magnetic head. A known disk drive structuredesigned as a countermeasure for this problem includes an FDB having ashaft, one end of which is fixed to a base. In the known structure, theshaft less vibrates so that the tracing of a recording track by amagnetic head can be prevented from being unacceptably disturbed.

Such a disk drive structure typically includes two capillary sealsprovided at two ends or sides of an FDB respectively. When lubricant isinjected into the FDB via one of the capillary seals, air bubbles tendto be drawn thereinto via the other capillary seal. The lubricant cannot easily move into a narrow clearance in the FDB. Therefore, chargingthe FDB with the lubricant tends to take a long time. During alonger-time charging process, more air bubbles are drawn into the FDB.Air bubbles in the FDB reduce bearing stiffness. A reduction in bearingstiffness may cause an FDB malfunction. In the event that the amount ofthe lubricant in the FDB is insufficient, the life of the FDB may beshort. Accordingly, it is desirable to check the amount of the lubricantin the FDB.

PCT application publication number WO 96/25606 discloses a hydrodynamicbearing which takes a cylindrical shape. The hydrodynamic bearing has astationary shaft, one end of which is coupled to a thrust plate forminga part of a base. The hydrodynamic bearing has two capillary seals attwo ends thereof. One of the capillary seals is inverted while the otheris non-inverted. The two capillary seals have upwardly-facing open endsrespectively.

SUMMARY OF THE INVENTION

It is a first object of this invention to provide a fluid dynamicbearing unit designed so that charging the unit with lubricant can beeasily performed in a short time, and air bubbles can be prevented frombeing drawn into the unit, and that the amount of the lubricant in theunit can be easily checked.

It is a second object of this invention to provide a disk drive deviceincluding the foregoing fluid dynamic bearing unit.

It is a third object of this invention to provide a method ofmanufacturing the foregoing fluid dynamic bearing unit.

A first aspect of this invention provides a fluid dynamic bearing unitcomprising a shaft; a first sleeve accommodating at least a portion ofthe shaft and being rotatable relative to the shaft; a first flangefixed to the shaft and spaced from a first end surface of the firstsleeve; a second flange fixed to the shaft and spaced from a second endsurface of the first sleeve opposite to the first end surface thereof; asecond sleeve fixed to the first sleeve and extending around the firstsleeve; a first annular member fixed to a first end of the second sleeveand surrounding the first flange; a second annular member fixed to thesecond flange and surrounding a portion of the second sleeve; a pair ofradial dynamic pressure grooves provided in at least one of an outercircumferential surface of the shaft and an inner circumferentialsurface of the first sleeve and spaced in an axial direction; a firstthrust dynamic pressure groove provided in at least one of the firstflange and the first end surface of the first sleeve; a second thrustdynamic pressure groove provided in at least one of the second flangeand the second end surface of the first sleeve; a first capillary sealincluding a first clearance between the first flange and the firstannular member and having a first open end, wherein the first clearanceat a first position is wider as the first position moves toward thefirst open end; a second capillary seal including a second clearancebetween the second annular member and the portion of the second sleeveand having a second open end, wherein the second clearance at a secondposition is wider as the second position moves toward the second openend; and lubricant provided in the radial dynamic pressure grooves, thefirst and second thrust dynamic pressure grooves, and the first andsecond capillary seals; wherein the second annular member and the secondsleeve are designed so that the lubricant in the second clearance in thesecond capillary seal can be viewed from a point in a radial positionwhich is outward of the second sleeve as seen in an axial direction.

A second aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit wherein the second sleeve andthe first annular member are integral with each other.

A third aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit wherein the second flange andthe second annular member are integral with each other.

A fourth aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit wherein an outercircumferential surface of the second sleeve has a recessed areadefining a portion of the second capillary seal.

A fifth aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit wherein a radial position ofthe second capillary seal is outward of that of the first capillary sealas seen in an axial direction.

A sixth aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit wherein the second capillaryseal extends around one of the radial dynamic pressure grooves.

A seventh aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit wherein at least one of thefirst and second annular members has an inner circumferential surfacewhich is tapered in an inward direction.

An eighth aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit further comprising acommunication passage including an axial groove formed in at least oneof the first sleeve and the second sleeve.

A ninth aspect of this invention is based on the eighth aspect thereof,and provides a fluid dynamic bearing unit wherein the first flangeradially extends to a position which is outward of a radial position ofan end of the communication passage as seen in an axial direction.

A tenth aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit wherein an outside diameter ofthe first annular member is smaller than that of the second annularmember.

An eleventh aspect of this invention is based on the first aspectthereof, and provides a fluid dynamic bearing unit wherein a volume ofthe second capillary seal is equal to 70%-130% of that of the firstcapillary seal.

A twelfth aspect of this invention is based on the first aspect thereof,and provides a fluid dynamic bearing unit wherein the second flange andthe second sleeve define a third clearance therebetween, and the secondcapillary seal includes the third clearance.

A thirteenth aspect of this invention is based on the first aspectthereof, and provides a fluid dynamic bearing unit wherein the secondflange is formed by pressing.

A fourteenth aspect of this invention is based on the first aspectthereof, and provides a fluid dynamic bearing unit wherein the firstsleeve and the second sleeve are integral with each other.

A fifteenth aspect of this invention is based on the first aspectthereof, and provides a fluid dynamic bearing unit wherein the firstcapillary seal and the second capillary seal are designed so that thelubricant in the first clearance in the first capillary seal and thelubricant in the second clearance in the second capillary seal can beviewed from a substantially same point in a radial position which isoutward of the second sleeve as seen in an axial direction.

A sixteenth aspect of this invention provides a disk drive devicecomprising a fluid dynamic bearing unit, a hub connected with the fluiddynamic bearing unit, and a base connected with the fluid dynamicbearing unit. The fluid dynamic bearing unit comprises a shaft fixed tothe base; a first sleeve accommodating at least a portion of the shaftand being rotatable relative to the shaft; a first flange fixed to theshaft and spaced from a first end surface of the first sleeve; a secondflange fixed to the shaft and spaced from a second end surface of thefirst sleeve opposite to the first end surface thereof; a second sleevefixed to the first sleeve and extending outward of the first sleeve, thesecond sleeve being securely connected with the hub; a first annularmember fixed to a first end of the second sleeve and surrounding thefirst flange; a second annular member fixed to the second flange andsurrounding a portion of the second sleeve; a pair of radial dynamicpressure grooves provided in at least one of an outer circumferentialsurface of the shaft and an inner circumferential surface of the firstsleeve and spaced in an axial direction; a first thrust dynamic pressuregroove provided in at least one of the first flange and the first endsurface of the first sleeve; a second thrust dynamic pressure grooveprovided in at least one of the second flange and the second end surfaceof the first sleeve; a first capillary seal including a first clearancebetween the first flange and the first annular member and having a firstopen end, wherein the first clearance at a first position is wider asthe first position moves toward the first open end; a second capillaryseal including a second clearance between the second annular member andthe portion of the second sleeve and having a second open end, whereinthe second clearance at a second position is wider as the secondposition moves toward the second open end; and lubricant provided in theradial dynamic pressure grooves, the first and second thrust dynamicpressure grooves, and the first and second capillary seals; wherein thesecond annular member and the second sleeve are designed so that thelubricant in the second clearance in the second capillary seal can beviewed from a point in a radial position which is outward of the secondsleeve as seen in an axial direction.

A seventeenth aspect of this invention is based on the sixteenth aspectthereof, and provides a disk drive device further comprising a coildisposed in a space between the hub and the base, the hub having aportion axially aligning with the coil, the base having a portionaxially aligning with the coil, wherein the portion of the hub isgreater in axial dimension than the portion of the base.

An eighteenth aspect of this invention is based on the sixteenth aspectthereof, and provides a disk drive device wherein the shaft has aportion directly connected with the base, and an axial dimension of theportion of the shaft is equal to 20% or more of an overall axial lengthof the shaft.

A nineteenth aspect of this invention is based on the sixteenth aspectthereof, and provides a disk drive device wherein the hub has an innersurface including a tapered surface.

A twentieth aspect of this invention is based on the sixteenth aspectthereof, and provides a disk drive device further comprising a covermember concealing the first open end of the first capillary seal.

A twenty-first aspect of this invention is based on the twentieth aspectof this invention, and provides a disk drive device wherein the covermember and the hub are integral with each other.

A twenty-second aspect of this invention provides a method ofmanufacturing a fluid dynamic bearing unit which comprises the steps offixing a first flange to an outer circumferential surface of an end of ashaft; fixing an outer circumferential surface of a first sleeve to aninner circumferential surface of a second sleeve with a first annularmember, wherein radial dynamic pressure grooves are formed between thefirst and second sleeves, and first and second thrust dynamic pressuregrooves are formed in end surfaces of the first sleeve; inserting an endof the shaft which is remote from the first flange into a central boreof the first sleeve until the first flange and the first annular memberare radially opposed to each other and said end of the shaft moves outof the central bore of the first sleeve, wherein a predeterminedclearance for a first capillary seal is defined between the first flangeand the first annular member; fixing a second flange with a secondannular member to the shaft at a position such that an outercircumferential surface of an end of the second sleeve and an innercircumferential surface of the second annular member are opposed to eachother, and thereby obtaining a fluid dynamic bearing unit withoutlubricant, wherein a predetermined clearance for a second capillary sealis defined between the second sleeve and the second annular member;placing the fluid dynamic bearing unit without lubricant in a vacuumchamber; applying lubricant to first and second portions of the fluiddynamic bearing unit without lubricant in the vacuum chamber, the firstportion including a region adjoining the clearance between the firstflange and the first annular member, the second portion including aregion adjoining the clearance between the second sleeve and the secondannular member; introducing air into the vacuum chamber and moving theapplied lubricant into the fluid dynamic bearing unit without lubricantby the introduced air so that the lubricant occupies the radial dynamicpressure grooves, the first and second thrust dynamic pressure grooves,at least a deep portion of the clearance between the first flange andthe first annular member for the first capillary seal, and at least adeep portion of the clearance between the second sleeve and the secondannular member for the second capillary seal, wherein the first andsecond capillary seals have open ends designed to allow surfaces of thelubricant in the first and second capillary seals to be viewed from asubstantially same point in a radial position which is outward of thesecond sleeve as seen in an axial direction; and checking positions ofsurfaces of the lubricant in the first and second capillary seals whileoptically accessing the surfaces of the lubricant through the open endsof the first and second capillary seals.

This invention has the following advantages. The lubricant is injectedinto the fluid dynamic bearing unit simultaneously via the clearances ofthe first and second capillary seals. Therefore, the injection of thelubricant into the fluid dynamic bearing unit can be easily performedand be completed in a short time. Furthermore, it is possible to preventair bubbles from being drawn into the fluid dynamic bearing unit duringthe injection of the lubricant thereinto. In addition, it is easy tocheck the amount of the lubricant in the fluid dynamic bearing unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of a disk drive device in an embodiment of thisinvention with a top cover removed for showing an internal structure ofthe device.

FIG. 1B is a side view of the disk drive device in FIG. 1A.

FIG. 2 is a sectional view of a main portion of the disk drive devicewhich is taken along the A-B line of FIG. 1A.

FIG. 3 is a longitudinal section view of a fluid dynamic bearing unit(FDB) in the disk drive device of FIGS. 1A, 1B, and 2.

FIG. 4A is a longitudinal section view of a combination of FDB parts ina state where a first flange is fixed to a shaft during the manufactureof the FDB.

FIG. 4B is a longitudinal section view of a combination of FDB parts ina state where a first sleeve and a second sleeve are fixed to each otherduring the manufacture of the FDB.

FIG. 4C is a longitudinal section view of a combination of FDB parts ina state where the shaft with the first flange is inserted into thesecond sleeve during the manufacture of the FDB.

FIG. 4D is a longitudinal section view of a combination of FDB parts ina state where a second flange is fixed to the shaft during themanufacture of the FDB.

FIG. 4E is a longitudinal section view of a combination of FDB parts ina state where air is expelled from an interior of the combination duringthe manufacture of the FDB.

FIG. 4F is a longitudinal section view of a combination of FDB parts ina state where lubricant is applied to first and second portions of thecombination which includes a region between the first flange and a firstannular member, and a region between the second sleeve and a secondannular member during the manufacture of the FDB.

FIG. 4G is a longitudinal section view of a combination of FDB parts ina state where the lubricant is drawn into the interior of thecombination during the manufacture of the FDB.

FIG. 4H is a longitudinal section view of a combination of FDB parts ina state where a laser beam is applied to interiors of first and secondcapillary seals to check the heights of lubricant surfaces in the firstand second capillary seals during the manufacture of the FDB.

FIG. 5A is a longitudinal section view of a combination of parts in astate where a yoke and a magnet are fixed to an inner circumferentialsurface of a hub during the assembly of the disk drive device.

FIG. 5B is a longitudinal section view of a combination of parts in astate where an outer circumferential surface of the FDB is fixed to aninner circumferential surface of the hub during the assembly of the diskdrive device.

FIG. 5C is a longitudinal section view of a combination of parts in astate where the FDB shaft is fixed to a base during the assembly of thedisk drive device.

FIG. 5D is a longitudinal section view of a combination of parts in astate where recording disks, spacers, and a clamper are fixed to thehub, and a top cover is securely located in position during the assemblyof the disk drive device.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1A, 1B, and 2, a disk drive device 100 in anembodiment of this invention includes a top cover 2, a base 12, and afluid dynamic bearing unit (FDB) 50.

The FDB 50 takes an approximately cylindrical body, and includes acolumnar shaft 11 coaxial with and centered at the body. One end (thelower end in FIG. 2) of the shaft 11 is fixed to the base 12. The otherend (the upper end in FIG. 2) of the shaft 11 is formed with anaxially-extending threaded hole 11A. The other end of the shaft 11 isfixed to the top cover 2 by a screw 13 having a head in contact with thetop cover 2 and extending into the threaded hole 11A to engage with theshaft 11.

As best shown in FIG. 3, the FDB 50 includes the shaft 11, a firstsleeve 14, a first flange 15, a second flange 16, a second sleeve 17, afirst annular member 18, and a second annular member 19 substantiallycoaxial with each other. The first and second sleeves 14 and 17 areapproximately cylindrical. The first sleeve 14 is made of, for example,a copper-based alloy, and is formed by, for example, cutting ormachining. The shaft 11 is made of, for example, stainless steel such asSUS420J, and is formed by, for example, cutting or machining. The firstsleeve 14 coaxially accommodates an intermediate part of the shaft 11.The first sleeve 14 can rotate about the shaft 11. There is a clearancebetween the first sleeve 14 and the shaft 11.

A pair of grooves 20 for radial dynamic pressure are formed in at leastone of the inner circumferential surface of the first sleeve 14 and theouter circumferential surface of the shaft 11. Thus, the radial dynamicpressure grooves 20 are defined between the first sleeve 14 and theshaft 11. The radial dynamic pressure grooves 20 are spaced in the axialdirection, and take, for example, a herringbone shape or pattern.

The first flange 15 is fixed to one end (the upper end in FIG. 3) of theshaft 11 by, for example, pressing. The first flange 15 coaxiallyextends around the shaft 11. One end surface of the first sleeve 14 andone end surface of the first flange 15 are opposed to each other whilebeing spaced from each other by a clearance. A first groove 21 forthrust dynamic pressure is formed in at least one of the opposed endsurfaces of the first sleeve 14 and the first flange 15. Thus, the firstthrust dynamic pressure groove 21 is defined between the first sleeve 14and the first flange 15. The first thrust dynamic pressure groove 21takes, for example, a spiral shape or a herringbone shape (pattern).

The second flange 16 is fixed to the shaft 11 by, for example, pressing.The second flange 16 coaxially extends around the shaft 11. The otherend surface of the first sleeve 14 and one end surface of the secondflange 16 are opposed to each other while being spaced from each otherby a clearance. A second groove 22 for thrust dynamic pressure is formedin at least one of the opposed end surfaces of the first sleeve 14 andthe second flange 16. Thus, the second thrust dynamic pressure groove 22is defined between the first sleeve 14 and the second flange 16. Thesecond thrust dynamic pressure groove 22 takes, for example, a spiralshape or a herringbone shape (pattern).

The second sleeve 17 coaxially accommodates the first sleeve 14, and isfixed thereto by, for example, bond. The first annular member 18 isfixed to an end surface of the second sleeve 17 which is close to thefirst flange 15. The first annular member 18 coaxially surrounds thefirst flange 15. There is a cylindrical clearance between the firstflange 15 and the first annular member 18. The first flange 15 and thefirst annular member 18 form a first capillary seal 24 including theforegoing clearance. The first capillary seal 24 has an open end (anupper end in FIG. 3). The clearance in the first capillary seal 24 istapered such that the cross section of the clearance at an axialposition is wider as the axial position moves toward the open end of theseal 24. In FIG. 3, the open end of the first capillary seal 24 facesaxially upward. The first capillary seal 24 exhibits capillary actionand thereby prevents leakage of lubricant 23 from the FDB 50.

The second annular member 19 is coaxially fixed to an outercircumferential edge of the second flange 16. The second annular member19 coaxially surrounds an end portion (a lower end portion in FIG. 3) ofthe second sleeve 17. There is a cylindrical clearance between thesecond annular member 19 and the end portion of the second sleeve 17.The second annular member 19 and the end portion of the second sleeve 17form a second capillary seal 25 including the foregoing clearance. Thesecond capillary seal 25 has an open end (an upper end in FIG. 3). Theclearance in the second capillary seal 25 is tapered such that the crosssection of the clearance at an axial position is wider as the axialposition moves toward the open end of the seal 25. In FIG. 3, the openend of the second capillary seal 25 faces axially upward. The secondcapillary seal 25 exhibits capillary action and thereby prevents leakageof the lubricant 23 from the FDB 50.

The radial dynamic pressure grooves 20, the first and second thrustdynamic pressure grooves 21 and 22, and the clearances in the first andsecond capillary seals 24 and 25 communicate with each other, and form acontinuous space charged with the lubricant 23. There is a first surface26 of the lubricant 23 (that is, an interface between the lubricant 23and air) at a midway position within the clearance of the firstcapillary seal 24. There is a second surface 27 of the lubricant 23(that is, an interface between the lubricant 23 and air) at a midwayposition within the clearance of the second capillary seal 25. Thesecond annular member 19 and the second sleeve 17 are shaped andrelatively arranged so that the lubricant 23 in the clearance of thesecond capillary seal 25 can be viewed from a point in a radial positionoutward of the second sleeve 17 as seen in the axial direction via theopen end of the second capillary seal 25. Thus, a straight path of lightpropagation can extend from the above-indicated view point to thelubricant 23 in the clearance of the second capillary seal 25 throughthe open end thereof without meeting the walls of the body of the FDB50. In this case, it is easy to optically check the position of thesurface 27 of the lubricant 23 in the second capillary seal 25 from thatview point. With reference to FIG. 3, the outer circumferential surfaceof the second sleeve 17 may have a recess 17A located immediately abovethe open end of the second capillary seal 25 for ensuring an opticalaccess to the lubricant 23 in the second capillary seal 25 from theabove-indicated view point. Preferably, the first and second capillaryseals 24 and 25 are designed so that the lubricant surfaces 26 and 27can be viewed in a substantially same direction or viewed from asubstantially same point whose radial position is outward of the outercircumference of the second sleeve 17 as seen in the axial direction viathe open ends of the seals 24 and 25. In FIG. 3, this view point is, forexample, axially and upwardly distant from the first and secondcapillary seals 24 and 25, and is radially distant from the axis of theshaft 11. The lubricant surfaces 26 and 27 in the first and secondcapillary seals 24 and 25 can be optically accessed from theabove-indicated view point via the open ends of the seals 24 and 25. Inother words, straight paths for light propagation can extend from thatview point to the lubricant surfaces 26 and 27 in the first and secondcapillary seals 24 and 25 through the open ends thereof without meetingthe walls of the body of the FDB 50. Accordingly, both the lubricantsurfaces 26 and 27 in the first and second capillary seals 24 and 25 canbe checked by an optical position-detecting device or an opticaldistance-measuring device located at the above-indicated view point. Anexample of the optical distance-measuring device is a laserdistance-measuring device.

The FDB 50 is manufactured in a method including an assembling procedureand a lubricant injecting procedure. The FDB 50 is assembled and thelubricant 23 is injected or impregnated thereinto by a sequence of steps(1)-(9) mentioned below.

(1) With reference to FIG. 4A, a shaft 11 and a first flange 15 areprepared. An end of the shaft 11 has a threaded hole (the threaded hole11A in FIGS. 2 and 3). The first flange 15 is fixed to the outercircumferential surface of this end of the shaft 11 at a predeterminedposition by, for example, pressing.

(2) With reference to FIG. 4B, a first sleeve 14 taking a predeterminedshape is prepared. Radial dynamic pressure grooves 20 are formed in theinner circumferential surface of the first sleeve 14. First and secondthrust dynamic pressure grooves 21 and 22 are formed in the end surfacesof the first sleeve 14. Preferably, the formation of the grooves 20, 21,and 22 utilizes machining, etching, or processing. Thereafter, a secondsleeve 17 is prepared. A first annular member 18 is fixed to an endsurface of the second sleeve 17. The first sleeve 14 is fitted in thesecond sleeve 17 with the first annular member 18. The outercircumferential surface of the first sleeve 14 is fixed to the innercircumferential surface of the second sleeve 17 by, for example, bond.

(3) With reference to FIG. 4C, the end of the shaft 11 which is remotefrom the first flange 15 is inserted into the central bore of the firstsleeve 14 until the first flange 15 and the first annular member 18 areradially opposed to each other and the end of the shaft 11 moves out ofthe central bore of the first sleeve 14. There is a predeterminedclearance between the first flange 15 and the first annular member 18for a first capillary seal 24.

(4) With reference to FIG. 4D, a second flange 16 is prepared. A secondannular member 19 is coaxially fixed to an outer circumferential edge ofthe second flange 16. The second flange 16 with the second annularmember 19 is mounted on the exposed portion of the shaft 11 at apredetermined position such that the outer circumferential surface of anend of the second sleeve 17 and the inner circumferential surface of thesecond annular member 19 are opposed to each other. There is apredetermined clearance between the second sleeve 17 and the secondannular member 19 for a second capillary seal 25. In this way, an FDB 50without lubricant 23 is obtained.

(5) With reference to FIG. 4E, the FDB 50 without lubricant 23 is placedin a vacuum chamber (not shown). The vacuum chamber is evacuated toremove air from the interior of the FDB 50. At this time, the pressurein the vacuum chamber is set to, for example, 100 Pa or less.

(6) With reference to FIG. 4F, in the vacuum chamber, lubricant 23 isapplied to first and second portions of the FDB 50 through the use of anozzle or nozzles 28. The first portion includes a region adjoining theopen end of the clearance between the first flange 15 and the firstannular member 18 for the first capillary seal 24. The second portionincludes a region adjoining the open end of the clearance between thesecond sleeve 17 and the second annular member 19 for the secondcapillary seal 25.

(7) With reference to FIG. 4G, air is introduced into the vacuum chamberto return the pressure therein to substantially the atmosphericpressure. The introduction of air develops a pressure difference movingthe applied lubricant 23 into the FDB 50. As a result, the lubricant 23occupies the radial dynamic pressure grooves 20, and the first andsecond thrust dynamic pressure grooves 21 and 22. Furthermore, thelubricant 23 occupies at least a deep portion of the clearance betweenthe first flange 15 and the first annular member 18 for the firstcapillary seal 24 and at least a deep portion of the clearance betweenthe second sleeve 17 and the second annular member 19 for the secondcapillary seal 25.

(8) With reference to FIG. 4H, the heights (positions) of the first andsecond surfaces 26 and 27 of the lubricant 23 in the first and secondcapillary seals 24 and 25 are detected through the use of a laserdistance-measuring device 29. In this case, the laser distance-measuringdevice 29 applies a laser beam 30 to the first and second lubricantsurfaces 26 and 27, and receives a reflected beam therefrom. A decisionis made as to whether or not the detected heights of the first andsecond lubricant surfaces 26 and 27 are in respective predeterminedranges meaning desired ranges relative to the FDB body. When thedetected heights of the first and second lubricant surfaces 26 and 27are in the desired ranges, the FDB 50 is regarded as being completed.

(9) In the event that the detected heights of the first and secondlubricant surfaces 26 and 27 are not in the desired ranges, the steps(5)-(8) are repeated to replenish the lubricant 23 in the FDB 50.

It should be noted that the step (6) may be modified as follows. Themodified step (6) does not apply lubricant 23 to the second portion ofthe FDB 50 which includes the region adjoining the open end of theclearance between the second sleeve 17 and the second annular member 19for the second capillary seal 25. In this case, during the step (7), airis drawn into the FDB 50 via the clearance between the second sleeve 17and the second annular member 19 for the second capillary seal 25 sothat a sufficient amount of the lubricant 23 is not moved into the FDB50 through the clearance between the first flange 15 and the firstannular member 18 for the first capillary seal 24. In addition, airbubbles tend to be drawn into the lubricant 23 within the FDB 50. Tocharge the FDB 50 sufficiently with the lubricant 23, the steps (5)-(8)are repeated a large number of times. Accordingly, sufficiently chargingthe FDB 50 with the lubricant 23 takes a long time, and the workefficiently of the charging is relatively low. In view of these points,it is preferable to apply lubricant 23 also to the second portion of theFDB 50 which includes the region adjoining the open end of the clearancebetween the second sleeve 17 and the second annular member 19 for thesecond capillary seal 25.

The second sleeve 17 and the first annular member 18 may be integralwith each other. Furthermore, the second flange 16 and the secondannular member 19 may be integral with each other. In these cases, thenumber of parts of the FDB 50 is smaller so that the efficiency ofassembling the FDB 50 is higher.

Each of the second sleeve 17 and the first annular member 18 is made ofa material selected from, for example, various metal materials andplastic materials. Preferably, each of the second sleeve 17 and thefirst annular member 18 is made of brass, and is formed by cutting ormachining. In this case, it is possible to provide the advantage thatthe cutting or the machining is easy, and the accuracy thereof is high.The second sleeve 17 and the first annular member 18 may be plated. Itis preferable to subject the second sleeve 17 and the first annularmember 18 to electroless nickel plating since a high antirust effect anda high hardness are attained.

Each of the second flange 16 and the second annular member 19 is made ofa material selected from, for example, various metal materials andplastic materials. Preferably, each of the second flange 16 and thesecond annular member 19 is made of stainless steel such as SUS303, andis formed by cutting or machining. In this case, it is possible toprovide the advantage that the cutting or the machining is easy, and theaccuracy thereof is high. Each of the second flange 16 and the secondannular member 19 may be made of stainless steel such as SUS304, and maybe formed by pressing. In this case, it is possible to provide theadvantage that forming the second flange 16 and the second annularmember 19 takes a short time.

As the outside diameter of the second capillary seal 25 is greater, theinside diameter of a stator core is greater and the developed rotationaldrive force is weaker. As best shown in FIG. 3, a radially-extending andcircumferentially-extending recess 17A may be formed in the outercircumferential surface of the second sleeve 17. In this case, it ispreferable to form the second capillary seal 25 at the outercircumferential surface of the second sleeve 17 which defines the recess17A since the outside diameter of the resultant seal 25 is smaller.

In the case where the second capillary seal 25 extends radially inwardof the first capillary seal 24 as seen in the axial direction, thenozzle 28 (see FIG. 4F) for applying lubricant 23 to the second portionof the FDB 50 tends to interfere with the first annular member 18. Inview of this point, it is preferable that the second capillary seal 25extends radially outward of the first capillary seal 24 as seen in theaxial direction. In this case, it is possible to provide the advantagethat as shown in FIG. 4F, the applications of lubricant 23 for the firstand second capillary seals 24 and 25 can be implemented in similardirections respectively.

In the case where the second capillary seal 25 and the radial dynamicpressure groove 20 closer thereto occupy different places in the axialdirection, the FDB 50 is axially thick. In view of this point, it ispreferable to locate the second capillary seal 25 coaxially around theradial dynamic pressure groove 20. In this case, it is possible toprovide the advantage that the FDB 50 is axially thin.

In operation of the FDB 50, the first and second sleeves 14 and 17 andthe first annular member 18 rotate about the shaft 11 while movingrelative to the first and second flanges 15 and 16 and the secondannular member 19. During rotation of the first and second sleeves 14and 17 and the first annular member 18, a centrifugal force is appliedto the lubricant 23 in the FDB 50. Preferably, at least one of the innercircumferential surfaces of the first and second annular members 18 and19 is tapered to prevent the escape of the lubricant 23 from the FDB 50due to the applied centrifugal force. In this case, the taper is suchthat the inside diameter of the first or second annular member 18 or 19at an axial position decreases as the axial position moves toward theopen end of the first or second capillary seal 24 or 25. In view of thelubricant-escape-preventing performance and the easy formation of thefirst and second annular members 18 and 19, a preferable range of theangle of the taper in at least one of the inner circumferential surfacesof the members 18 and 19 is between 0.5° and 10°. A more preferablerange is between 3° and 8°.

Preferably, an axially-extending communication passage 32 is provided inthe first sleeve 14 or at the boundary between the first and secondsleeves 14 and 17. The ends of the communication passage 32 open at theend surfaces of the first sleeve 14 or at the ends of the boundarybetween the first and second sleeves 14 and 17. The communicationpassage 32 connects the lubricant containing regions partially definedby the two end surfaces of the first sleeve 14. The communicationpassage 32 is filled with the lubricant 23. Accordingly, thecommunication passage 32 reduces the difference between the pressuresapplied to the two end surfaces of the first sleeve 14.

The communication passage 32 may include an axially-extending groove 31formed in at least one of the outer circumferential surface of the firstsleeve 14 and the inner circumferential surface of the second sleeve 17.In this case, it is easy to remove burrs caused during the formation ofthe groove 31.

Preferably, the outer circumference of the first flange 15 extendsradially outward of the nearby open end of the communication passage 32as seen in the axial direction. In this case, the first flange 15 blocksthe movement of the lubricant 23 from the communication passage 32 whena shock is applied to the FDB 50. For example, the outside diameter ofthe first flange 15 is greater than that of the first sleeve 14.

In the case where the outside diameter of the first annular member 18 isgreater than that of the second annular member 19, the laser beam 30tends to interfere with the first annular member 18 during an attempt tocheck the lubricant surface 27 in the second capillary seal 25 at thestep (8) of FIG. 4H. Preferably, the outside diameter of the firstannular member 18 is smaller than that of the second annular member 19.In this case, it is easy to check the lubricant surface 27 in the secondcapillary seal 25.

When the lubricant 23 disappears from the first or second capillary seal24 or 25 due to evaporation, the life of the FDB 50 expires. A smallerinitial amount of the lubricant in the first or second capillary seal 24or 25 causes a shorter life of the FDB 50. Preferably, thelubricant-containing volume or capacity of the second capillary seal 25is equal to 70%-130% of that of the first capillary seal 24. In thiscase, equal amounts of lubricant 23 are discharged from equal-sizenozzles 28 onto the first and second portions of the FDB 50 respectivelyat the step (6) of FIG. 4F. Accordingly, it is easy to manage theapplied amounts of lubricant 23. It should be noted that an error in theapplied amounts of lubricant 23 is generally equal to ±30% even when theequal-size nozzles 28 are used.

A greater lubricant-containing volume (capacity) of the second capillaryseal 25 is desirable. The second capillary seal 25 may include theclearance defined between the opposed surfaces of the second flange 16and the second sleeve 17. In this case, a greater lubricant-containingvolume of the second capillary seal 25 is obtained.

Preferably, the surface of the second flange 16 which opposes the nearbyend surface of the second sleeve 17 is flat. Preferably, the end surfaceof the second sleeve 17 which opposes the nearby surface of the secondflange 16 is tapered such that the clearance therebetween at a radialposition is wider as the radial position moves outward. The clearancebetween the opposing surfaces of the second flange 16 and the secondsleeve 17 is filled with the lubricant 23. This clearance also exhibitscapillary action that prevents leakage of the lubricant 23 from the FDB50. The clearance between the second sleeve 17 and the second annularmember 19 and the clearance between the second flange 16 and the secondsleeve 17 constitute an interior of the second capillary seal 25.

The first sleeve 14 and the second sleeve 17 may be integral with eachother. In this case, parts of the FDB 50 can be made by fewer steps.Each of the first and second sleeves 14 and 17 is made of a materialselected from, for example, various metal materials and plasticmaterials. Preferably, each of the first and second sleeves 14 and 17 ismade of brass, and is formed by cutting or machining. In this case, itis possible to provide the advantage that the cutting or the machiningis easy, and the accuracy thereof is high. The first and second sleeves14 and 17 may be plated. It is preferable to subject the first andsecond sleeves 14 and 17 to electroless nickel plating since a highantirust effect and a high hardness are attained.

With reference back to FIGS. 1A, 1B, and 2, the disk drive device 100includes a hub 10, the base 12, and the FDB 50. The hub 10 is separatefrom the second sleeve 17 of the FDB 50. The hub 10 has a central borein which at least a part of the FDB 50 is placed. The base 12 isseparate from the second flange 16 of the FDB 50.

The hub 10 is made of, for example, aluminum, and is formed by, forexample, cutting or machining. The hub 10 has an outer circumferentialsurface including first and second cylindrical portions. In FIG. 2, thefirst cylindrical portion of the hub 10 coaxially extends below thesecond cylindrical portion thereof. The first cylindrical portion of thehub 10 engages with the inner circumference of a recording disk 1 or theinner circumferences of recording disks 1. The second cylindricalportion of the hub 10 engages with the inner circumference of a clamper39. One end (the lower end in FIG. 2) of the hub 10 near the firstcylindrical portion may have an outwardly-projecting annular ledge orshelf.

The hub 10 has an inner circumferential surface including third andfourth cylindrical portions. The diameter of the fourth cylindricalportion of the hub 10 is greater than that of the third cylindricalportion thereof. In FIG. 2, the fourth cylindrical portion of the hub 10coaxially extends below the third cylindrical portion thereof. The thirdcylindrical portion of the hub 10 defines the central bore thereof intowhich the FDB 50 extends. The third cylindrical portion of the hub 10 isfixed to the outer circumferential surface of the second sleeve 17 inthe FDB 50 by, for example, bond. The outer circumferential surface of acylindrical yoke 33 is fixed to the fourth cylindrical portion of thehub 10 by, for example, pressing and bond. The yoke 33 is made of, forexample, a soft-magnetic steel sheet, and is formed by, for example,pressing. The yoke 33 may be plated to prevent corrosion.

A ring-shaped magnet 34 is coaxially fixed to the inner circumferentialsurface of the yoke 33. The magnet 34 is made of, for example, amaterial containing a rare-earth material such as Nd—Fe—B(neodymium-iron-boron). The magnet 34 may have, for example, eightdriving-purpose magnetic poles spaced along the circumferentialdirection of the inner circumferential surface thereof.

The base 12 may have a recess 12A and a wall portion 12B defining theedge of the recess 12A. The base 12 is made of, for example, aluminum,and is formed by, for example, die casting to produce an intermediatemember and cutting or machining the intermediate member into a desiredshape. It is preferable to provide an electrodeposition coat on thesurface of the base 12 since the coat prevents dust from falling offfrom the exposed base surface.

The base 12 has a hole 12C and an annular wall portion 12D extendingaround the hole 12C. The hole 12C is located in a central portion of thebase 12. A stator core 35 is fixed to the outer circumferential surfaceof the annular wall portion 12D by, for example, bond. The stator core35 may have a circular ring and twelve projections radially extendingfrom the circular ring. The projections of the stator core 35 areprovided with a 3-phase coil 36 formed by windings. The end of the shaft11 of the FDB 50 which is close to the second flange 16 fits into thehole 12C of the base 12, and is fixed to the base 12 by, for example,bond.

The inner circumferential surface of the magnet 34 and the outercircumferential surface of the stator core 35 are opposed to each other.There is a clearance of, for example, 0.5 mm between the innercircumferential surface of the magnet 34 and the outer circumferentialsurface of the stator core 35. The coil 36 is electrically connectedwith a given drive circuit (not shown). When the drive circuit causes a3-phase approximately-sinusoidal current to flow through the coil 36, arotational magnetic field is developed around the stator core 35. Thedeveloped rotational magnetic field and the driving-purpose magneticpoles of the magnet 34 interact with each other, thereby generating arotational drive force exerted on the magnet 34. Therefore, the magnet34 and also the hub 10 rotate about the shaft 11.

There is a combination in which doughnut-shaped recording disks 1coaxially alternate with doughnut-shaped spacers 38. The combination ofthe recording disks 1 and the spacers 38 may be placed on the ledge (theshelf) of the hub 10. The damper 39 takes a shape of an approximatelydoughnut. In FIG. 2, the clamper 39 may be placed on the uppermostrecording disk 1. The inner circumference of the clamper 39 may engagewith the second cylindrical portion of the hub 10. The clamper 39 may befixed to the hub 10 by a screw 40. In these cases, the clamper 39 andthe screw 40 cooperate as a mechanism for firmly holding the recordingdisks 1 on the hub 10.

As shown in FIG. 1, the disk drive device 100 may include a magnetichead 3, a drive unit 4 for the magnetic head 3, a control circuit (notshown) for the magnetic head 3 and the drive unit 4, and other parts.

As shown in FIGS. 1B and 2, the top cover 2 is located at an upperportion of the disk drive device 100. The top cover 2 is made of, forexample, an aluminum sheet or a steel plate, and is formed by, forexample, pressing. The top cover 2 may be plated to prevent corrosion.An edge of the top cover 2 may have a plurality of holes 2A foraccommodating screws 5. A central portion of the top cover 2 may have ahole 2B for accommodating the screw 13. Preferably, the top cover 2 isfixed to the end of the shaft 11 by the screw 13 which has a head incontact with the top cover 2 and which extends into the threaded hole11A through the hole 2B of the top cover 2 to engage with the shaft 11.As best shown in FIG. 1, the wall portion 12B of the base 12 has holes12F corresponding to the holes 2A in the top cover 2. The top cover 2may be fixed to the base 12 by the screws 5 which have heads in contactwith the top cover 2 and which extend into the respective holes 12F inthe base 12 through the respective holes 2A in the top cover 2 to engagewith the wall portion 12B of the base 12. Thus, in these cases, one endof the shaft 11 of the FDB 50 is directly fixed to the base 12 while theother end thereof is fixed to the base 12 through the top cover 2.Accordingly, the shaft 11 of the FDB 50 is stably and firmly supportedso that vibration of the shaft 11 is effectively suppressed. Therefore,it is possible to reduce a disturbance in the tracing of a recordingtrack on a currently-accessed recording disk 1 by the magnetic head 3which is caused by vibration of the shaft 11.

A method of assembling the disk drive device 100 includes a sequence ofsteps (1)-(4) mentioned below.

(1) With reference to FIG. 5A, a hub 10, a yoke 33, and a basic memberfor a magnet 34 are prepared. The outer circumferential surface of theyoke 33 is fixed to the fourth cylindrical portion of the hub 10 by, forexample, pressing and bond. The outer circumferential surface of themagnet basic member is fixed to the inner circumferential surface of theyoke 33 by, for example, bond. The magnet basic member is magnetized bya magnetizing device (not shown) so that the magnet basic member changesinto a magnet 34 having driving-purpose magnetic poles spaced along thecircumferential direction of the inner circumferential surface thereof.

(2) With reference to FIG. 5B, an FDB 50 is prepared. The FDB 50 isinserted into the central bore of the hub 10. The outer circumferentialsurface of the second sleeve 17 in the FDB 50 is fixed to the thirdcylindrical portion of the hub 10 by, for example, bond.

(3) With reference to FIG. 5C, a stator core 35 is prepared. A coil 36is formed on the projections of the stator core 35. The stator core 35is fixed to the outer circumferential surface of the annular wallportion 12D of a base 12 by, for example, bond. At the same time, an endof the shaft 11 of the FDB 50 which is close to the second flange 16 isinserted into the hole 12C of the base 12 and is fixed to the wall ofthe base 12 by, for example, bond.

(4) With reference to FIG. 5D, recording disks 1, spacers 38, a clamper39, and a top cover 2 are prepared. An alternation of the recordingdisks 1 and the spacers 38 is placed on the ledge (the shelf) of the hub10. The inner circumference of the clamper 39 is brought into engagementwith the second cylindrical portion of the hub 10, and is placed on theuppermost recording disk 1. The clamper 39 is fixed to the hub 10 by ascrew 40.

With reference back to FIG. 2, “X” denotes the axial dimension of aportion of the hub 10 which is located at a radial position equal tothat of the coil 36 and hence axially aligns with the coil 36, and “Y”denotes the axial dimension of a portion of the base 12 which is locatedat a radial position equal to that of the coil 36 and hence axiallyaligns with the coil 36. Preferably, the axial dimension “X” is greaterthan the axial dimension “Y”. In this case, it is possible to providethe advantage that the rocking-mode resonance frequency of the diskdrive device 100 is relatively high, and hence unwanted vibration can besuppressed. More preferably, the axial dimensions “X” and “Y” are chosento have the relation as “X>1.5Y”. In this case, a higher rocking-moderesonance frequency is obtained. The axial dimensions “X” and “Y” areset to, for example, 6.4 mm and 3.6 mm respectively.

In FIG. 2, “Z” denotes the axial dimension of a portion of the shaft 11which is directly connected with the base 12. Preferably, the axialdimension “Z” is equal to 20% or more of the overall axial length “L” ofthe shaft 11. In this case, it is possible to provide the advantage thatthe connection of the shaft 11 with the base 12 is stable and thestrength of the connection is sufficiently high. The axial dimension “Z”and the overall axial length “L” of the shaft 11 are set to, forexample, 5.4 mm and 23.4 mm respectively.

An inlet side or an entrance side (a lower side in FIG. 2) of the thirdcylindrical portion of the hub 10 may be formed with a tapered surface10A. During the step (2) of FIG. 5B, the tapered surface 10A serves as aguide when the FDB 50 is inserted into the third cylindrical portion ofthe hub 10. Specifically, the second sleeve 17 of the FDB 50 is easilyand smoothly moved into the third cylindrical portion of the hub 10while being guided by the tapered surface 10A. Therefore, the efficiencyof the work is relatively high, and the FDB 50 is accurately mounted onthe hub 50.

The end (the upper end in FIG. 2) of the hub 10 may be provided with anannular cover member 41 projecting radially inward. The cover member 41conceals the open end of the first capillary seal 24. The cover member41 blocks or reduces the escape of the lubricant 23 from the firstcapillary seal 24 which occurs when a shock is applied to the FDB 50.Preferably, the inside diameter of the cover member 41 is smaller thanthe outside diameter of the first flange 15. In this case, the covermember 41 can more effectively block or reduce the escape of thelubricant 23 from the first capillary seal 24.

The cover member 41 is made of a material selected from, for example,various metal materials and plastic materials. Preferably, the covermember 41 is made of stainless steel such as SUS303, and is formed bycutting or machining. In this case, the following advantages areprovided. The cutting or the machining is easy, and the accuracy thereofis high. The cover member 41 may be made of stainless steel such asSUS304, and may be formed by pressing. In this case, it is possible toprovide the advantage that forming the cover member 41 takes a shorttime.

The cover member 41 may be integral with the hub 10. In this case, it ispossible to provide the advantage that the cover member 41 and the hub10 can be made simultaneously by common steps.

What is claimed is:
 1. A disk drive device comprising: a basecomprising: a core supporting portion that supports a stator core aroundwhich a coil is wound; a bottom portion including a coil opposingportion facing with the coil in an axial direction; and acircumferential wall portion to which a top cover is to be fixed,wherein the core supporting portion, the bottom portion and thecircumferential wall portion are integrally formed one another; arotator comprising: a hub on which a recording disk is to be mounted;and a shaft enclosure, wherein the rotator is to be disposed between thebase and the top cover; a shaft assembly comprising: a shaft retained inthe shaft enclosure in a freely rotatable manner relative to the shaftenclosure; a projection comprising an opposing portion facing with afirst end face of the shaft enclosure in the axial direction; and aflange including an opposing surface facing with a second end face ofthe shaft enclosure in an axial direction, the flange being formedseparately from the core supporting portion, wherein the shaft assemblyhas one end fixed to the base, and has another end which is to be fixedto the top cover; and a pair of radial dynamic pressure generatinggrooves provided in at least one of the shaft assembly and the shaftenclosure, wherein the flange includes a portion facing with the shaftenclosure in the axial direction and having a smaller thickness than amaximum thickness of the coil opposing portion in the axial direction.2. The disk drive device according to claim 1, wherein: the shaftassembly includes a threaded hole having an opening at the upper endthereof; and the threaded hole is provided so as to traverse at leastone of the pair of dynamic pressure generating grooves in the axialdirection.
 3. The disk drive device according to claim 1, furthercomprising a thrust dynamic pressure generating groove provided in atleast one of the shaft enclosure, the flange and the projection.
 4. Thedisk drive device according to claim 1, wherein: the shaft enclosure isprovided with a radial-direction recess in an outer circumference of theshaft enclosure; and a tapered space is formed in an outer circumferenceof the radial-direction recess.
 5. The disk drive device according toclaim 1, wherein: the core supporting portion comprises an annular wallthat protrudes toward the rotator from the bottom portion in the axialdirection; the stator core comprises an annular portion; and an innercircumference of the annular portion comprises a portion contacting theannular wall, and a non-contact portion with the annular wall, thenon-contact portion being successively provided from the portioncontacting the annular wall at the hub side in the axial direction. 6.The disk drive device according to claim 1, wherein the shaft enclosurecomprises a plated surface.
 7. The disk drive device according to claim1, wherein the hub has a cover member covering a part of the projectionbeyond a part of the shaft enclosure in a radial direction.
 8. A diskdrive device comprising: a base comprising: a core supporting portionthat supports a stator core around which a coil is wound; and a bottomportion including a coil opposing portion facing with the coil in anaxial direction; wherein the core supporting portion and the bottomportion are integrally formed with each other; a rotator comprising: ahub on which a recording disk is to be mounted; and a shaft enclosure,wherein the rotator is to be disposed between the base and a top cover;a shaft assembly comprising: a shaft retained in the shaft enclosure ina freely rotatable manner relative to the shaft enclosure; a projectioncomprising an opposing portion facing with a first end face of the shaftenclosure in the axial direction; a flange including an opposing surfacefacing with a second end face of the shaft enclosure in an axialdirection, the flange being formed separately from the core supportingportion; and an annular enclosure fixed to the flange and encircling atleast a part of an outer circumference of the shaft enclosure, theannular enclosure receding in the axial direction relative to an end ofthe stator core at the hub side, wherein the shaft assembly has one endfixed to the base, and has another end which is to be fixed to the topcover; and a pair of radial dynamic pressure generating grooves providedin at least one of the shaft assembly and the shaft enclosure, whereinthe flange includes a portion facing the shaft enclosure in the axialdirection and having a smaller thickness than a maximum thickness of thecoil opposing portion.
 9. The disk drive device according to claim 8,wherein: the shaft assembly includes a thread hole having an opening atthe upper end thereof; and the threaded hole is provided so as totraverse at least one of the pair of dynamic pressure generating groovesin the axial direction.
 10. The disk drive device according to claim 8,further comprising a thrust dynamic pressure generating groove providedin at least one of the shaft enclosure, the flange and the projection.11. The disk drive device according to claim 8, wherein: the shaftenclosure is provided with a radial-direction recess in an outercircumference of the shaft enclosure; and a tapered space is formed inan outer circumference of the radial-direction recess.
 12. The diskdrive device according to claim 8, wherein: the core supporting portioncomprises an annular wall that protrudes toward the rotator from thebottom portion in the axial direction; the stator core comprises anannular portion; and an inner circumference of the annular portioncomprises a portion contacting the annular wall, and a non-contactportion with the annular wall, the non-contact portion beingsuccessively provided from the portion contacting the annular wall atthe hub side in the axial direction.
 13. The disk drive device accordingto claim 8, wherein the shaft enclosure comprises a plated surface. 14.The disk drive device according to claim 8, wherein the hub has a covermember covering a part of the projection beyond a part of the shaftenclosure in a radial direction.
 15. A disk drive device comprising: abase comprising: a core supporting portion that supports a stator corearound which a coil is wound; and a bottom portion including a coilopposing portion facing with the coil in an axial direction, wherein thecore supporting portion and the bottom portion are integrally formedwith each other; a rotator comprising: a hub on which a recording diskis to be mounted; and a shaft enclosure, wherein the rotator is to bedisposed between the base and a top cover; a shaft assembly comprising:a shaft retained in the shaft enclosure in a freely rotatable mannerrelative to the shaft enclosure; a projection comprising an opposingportion facing with a first end face of the shaft enclosure in the axialdirection; and a flange including an opposing surface facing with asecond end face of the shaft enclosure in an axial direction, the flangebeing formed separately from the core supporting portion, wherein theshaft assembly has one end fixed to the base, and has another end whichis to be fixed to the top cover; and a pair of radial dynamic pressuregenerating grooves provided in at least one of the shaft assembly andthe shaft enclosure, wherein: the flange includes a portion facing withthe shaft enclosure in the axial direction and having a smallerthickness than a maximum thickness of the coil opposing portion in theaxial direction; and a minimum thickness of the portion of the flangefacing with the shaft enclosure in the axial direction is smaller than amaximum thickness of the projection in the axial direction.
 16. The diskdrive device according to claim 15, wherein: the shaft assembly includesa threaded hole having an opening at the upper end thereof; and thethreaded hole is provided so as to traverse at least one of the pair ofdynamic pressure generating grooves in the axial direction.
 17. The diskdrive device according to claim 15, further comprising a thrust dynamicpressure generating groove provided in at least one of the shaftenclosure, the flange and the projection.
 18. The disk drive deviceaccording to claim 15, wherein: the shaft enclosure is provided with aradial-direction recess in an outer circumference of the shaftenclosure; and a tapered space is formed in an outer circumference ofthe radial-direction recess.
 19. The disk drive device according toclaim 15, wherein: the core supporting portion comprises an annular wallthat protrudes toward the rotator from the bottom portion in the axialdirection; the stator core comprises an annular portion; and an innercircumference of the annular portion comprises a portion contacting theannular wall, and a non-contact portion with the annular wall, thenon-contact portion being successively provided from the portioncontacting the annular wall at the hub side in the axial direction. 20.The disk drive device according to claim 15, wherein the shaft enclosurecomprises a plated surface.